From the viewpoint of reduction in use of electricity, the electrical machinery and appliances and mechanical parts used not only in the automobiles, but also in a variety of industries have been recently required to have higher efficiency. For this purpose, many attempts have been made, for example by reducing the weight of the parts, improving the structure, and the like. One of the methods for upgrading the efficiency in the automobile is considered to reduce the torque of the wheel bearing. Similarly, it becomes important for the grease for the wheel bearing to exhibit low torque performance over a wide temperature range extending from low to high temperatures.
The wheel bearing is disposed in proximity to the brake. When the brakes are continuously applied on a downward slope, for example, the wheel bearing may be exposed to high temperatures. In such a case, the surface initiated flaking may take place to shorten the anti-flaking life because the viscosity of the base oil used for the grease becomes too low to form a sufficient oil film. In addition, the circumstances of high temperature cause the grease to deteriorate, which may shorten the lubrication life. It is therefore necessary to choose a grease that can fully satisfy the anti-flaking life and the lubrication life.
The automobiles are often transported by train or truck. During the transportation, fretting may occur on the grease-applied parts to be lubricated due to minute vibrations induced by rail joints and rough roads. Particularly under the circumstances of low temperature, the grease cannot sufficiently flow into the parts to be lubricated because of low fluidity of the grease base oil, thereby causing the fretting as mentioned above. To solve the fretting problem, the parts have been improved, and the grease with improved anti-fretting properties has been employed.
For reducing the torque, there are methods of making the kinematic viscosity of base oil as low as possible, softening the grease, and decreasing the amount of grease applied to the mechanical parts. For example, JP 2000-198993 A proposes a grease composition where the base oil comprising an ester oil has a kinematic viscosity at 40° C. of 10 mm2/s or more.
The reduction in kinematic viscosity of the base oil can lower the torque because the resistance to stirring becomes low. However, a sufficient oil film cannot be formed, which may cause the surface initiated flaking and shorten the lubrication life due to the evaporation, so that the life of the bearing cannot be completed.
When the grease is softened, the resulting grease is apt to leak out, which will shorten the lubrication life, and cause the flaking due to insufficient oil film formation to result in the unsatisfactory anti-flaking life. When the amount of grease is decreased, the amount of grease fed to the parts to be lubricated becomes smaller, which cannot satisfy the lubrication life.
The fretting is a surface damage induced under a minute amplitude of vibration, to generate oxidized debris in the air, which is said to often cause considerable wear by the abrasive action (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., Ltd., issued on 28, Feb. 1998, pp. 201-203). To prevent the fretting, there are some proposals, for example, (1) to decrease the relative amplitude of sliding motion, (2) to separate the surfaces so as not to come in a direct contact therewith, and (3) to prevent the surfaces from adhering to each other by coating a phosphate film or the like on the contact surfaces or feeding a lubricating oil or grease to the contact surfaces (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., issued on 28, Feb. 1998, pp. 201-203). There is disclosed a grease composition with improved anti-fretting properties, comprising a urea thickener, a base oil, a phosphorothioate compound and an amine compound (JP 2008-239687 A). However, the fretting wear cannot be sufficiently prevented when the conditions become severer than conventionally supposed (i.e., at the temperature of as low as −30° C., under extremely minute amplitude of vibration). Any measures other than the prior arts mentioned above are needed for improving the fretting wear.